Machine tool and control apparatus of the machine tool

ABSTRACT

In a machine tool and a control apparatus thereof, a repetitive movement unit is configured so that the cutting tool performs one repetitive movement with respect to multiple relative rotations between the workpiece and the cutting tool and so that a rotation angle of the relative rotation between the workpiece and the cutting tool during relative movement at a second speed is smaller than a rotation angle of the relative rotation during relative movement at a first speed in one repetitive movement. This configuration limits degradation of machining efficiency when the cutting tool machines the workpiece by performing one repetitive movement with respect to multiple relative rotations between the workpiece and the cutting tool.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 15/506793,filed Feb. 27, 2017, now Patent No.

FIELD OF THE INVENTION

The present invention relates to a machine tool that machines aworkpiece while sequentially separating a chip generated during cuttingwork, and also relates to a control apparatus of the machine tool.

BACKGROUND OF THE INVENTION

Conventionally, a machine tool is known that includes a workpieceholding unit to hold a workpiece, a tool rest to hold a cutting tool forcutting the workpiece, a feeding unit to feed the cutting tool towardthe workpiece in a predetermined feeding direction via relative movementbetween the workpiece holding unit and the tool rest, a repetitivemovement unit to repetitively move the workpiece holding unit and thetool rest in a mutually relative manner by repeating the relativemovement in the feeding direction at a first speed and a second speedthat are mutually different, and a rotating unit to relatively rotatethe workpiece and the cutting tool, the machine tool being capable ofmachining the workpiece via the relative rotation between the workpieceand the cutting tool and via the feeding of the cutting tool toward theworkpiece with the repetitive movement. Examples of such machine toolsare described in Japanese Patent No. 5139591 (see particularly paragraph0039), and in Japanese Laid-Open Patent application No. H10-43901 (seeparagraph 0019).

When one of these machine tool machines a workpiece, a cutting workposition of a forward movement of a reciprocal vibration, which is oneexample of the repetitive movement, overlaps with a cutting workposition of a backward movement of the reciprocal vibration. Thus, thecutting tool performs an “air cut,” in which the cutting tool simplymoves without performing actual cutting during the backward movementbecause a part of the workpiece that is supposed to be cut in thebackward movement has already been cut in the forward movement. Due tothis, it is possible to machine a workpiece smoothly while sequentiallyseparating a chip generated from the workpiece during the cutting work.

When the reciprocal vibration is performed such that the cutting workposition of the forward movement overlaps the cutting work position ofthe backward movement, especially when the number of the reciprocalvibrations with respect to one rotation of a spindle that holds androtates a workpiece is less than one, i.e., when the cutting toolvibrates once with respect to multiple rotations of the spindle, it isdesirable to have a reciprocal vibration pattern that is capable oflimiting degradation of machining efficiency.

For example, suppose that, in a reciprocal vibration pattern in whichthe reciprocal vibration is performed once with respect to fourrotations of the spindle, which causes a tool edge of the cutting toolto trace a path in the form of a reciprocal vibration waveformillustrated in FIG. 6A, the path traced during a first rotation of thespindle, the path traced during a second rotation of the spindle, thepath traced during a third rotation of the spindle, and the path tracedduring a fourth rotation of the spindle are indicated by a, b, c, and d,respectively. If the cutting tool performs a forward movement during thefirst and second rotations and performs a backward movement during thethird and fourth rotations, the paths a to d with respect to the phaseof the spindle can be illustrated as shown in FIG. 6B. In this case, anair cut occurs after an intersection point CT between the path c and thepath d. This means that the air cut by the cutting tool occurs after thecutting work position of the backward movement overlaps the cutting workposition of the forward movement in the third rotation of the spindle,and continues until the end of the fourth rotation of the spindle. Thus,the air cut is performed for more than one rotation of the spindle,causing degradation of cutting efficiency.

Thus, it is an object of the present invention to address the aboveproblem of the conventional art by providing a machine tool that iscapable of limiting degradation of machining efficiency when a cuttingtool machines a workpiece by performing one repetitive movement withrespect to multiple relative rotations between the workpiece and thecutting tool, and by providing a control apparatus of the machine tool.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, the above problemis addressed by a machine tool comprising: a workpiece holding unit tohold a workpiece; a tool rest to hold a cutting tool for cutting theworkpiece; a feeding unit to feed the cutting tool toward the workpiecein a predetermined feeding direction via relative movement between theworkpiece holding unit and the tool rest; a repetitive movement unit torepetitively move the workpiece holding unit and the tool rest in amutually relative manner by repeating the relative movement in thefeeding direction at a first speed and a second speed that are mutuallydifferent; and a rotating unit to relatively rotate the workpiece andthe cutting tool, the machine tool being capable of machining theworkpiece via the relative rotation between the workpiece and thecutting tool and via the feeding of the cutting tool toward theworkpiece with the repetitive movement, wherein the repetitive movementunit is configured so that the cutting tool performs one repetitivemovement with respect to multiple relative rotations between theworkpiece and the cutting tool and so that a rotation angle of therelative rotation during the relative movement at the second speed issmaller than a rotation angle of the relative rotation during therelative movement at the first speed in one repetitive movement.

According to a second aspect of the present invention, the first speedis set faster than the second speed.

According to a third aspect of the present invention, a reference angleposition is set on the basis of a rotation angle position of therelative rotation at which a cutting work position of the relativemovement at the first speed and a cutting work position of the relativemovement at the second speed intersect each other, and the repetitivemovement unit is configured so that, during one rotation from thereference angle position after the relative movement at the first speedis performed for the length of a predetermined number of the relativerotations from the reference angle position, the relative movement atthe first speed switches to the relative movement at the second speedand the cutting work position of the relative movement at the secondspeed reaches the cutting work position of the relative movement at thefirst speed to complete one repetitive movement.

According to a fourth aspect of the present invention, the above problemis addressed by a control apparatus of a machine tool that includes: aworkpiece holding unit to hold a workpiece; a tool rest to hold acutting tool for cutting the workpiece; a feeding unit to feed thecutting tool toward the workpiece in a predetermined feeding directionvia relative movement between the workpiece holding unit and the toolrest; a repetitive movement unit to repetitively move the workpieceholding unit and the tool rest in a mutually relative manner byrepeating the relative movement in the feeding direction at a firstspeed and a second speed that are mutually different; and a rotatingunit to relatively rotate the workpiece and the cutting tool, themachine tool being capable of machining the workpiece via the relativerotation between the workpiece and the cutting tool and via the feedingof the cutting tool toward the workpiece with the repetitive movement,wherein the repetitive movement unit is configured so that the cuttingtool performs one repetitive movement with respect to multiple relativerotations between the workpiece and the cutting tool and so that arotation angle of the relative rotation during the relative movement atthe second speed of one repetitive movement is smaller than a rotationangle of the relative rotation during the relative movement at the firstspeed of the one repetitive movement.

According to the machine tool of the first or second aspect of thepresent invention, the rotation angle of the relative rotation duringthe relative movement at the second speed is smaller than the rotationangle of the relative rotation during the relative movement at the firstspeed in one repetitive movement. Thus, the workpiece can be cutefficiently while the repetitive movement due to the relative movementat the mutually different first and second speeds is performed.Particularly, it is possible to limit degradation of machiningefficiency when the repetitive movement constitutes a vibration.

According to the machine tool of the third aspect of the presentinvention, the time period of a so-called air cut, in which the cuttingtool leaves the workpiece and simply moves without performing actualcutting, during the relative movement at the second speed can be limitedto further increase the machining efficiency.

According to the control apparatus of the machine tool of the fourthaspect of the present invention, the same effects achieved by the firstaspect of the present invention can also be achieved by the controlapparatus of the machine tool.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a machine tool of anembodiment of the present invention.

FIG. 2 is a schematic diagram illustrating the relationship between acutting tool and a workpiece in the embodiment of the present invention.

FIG. 3 is a diagram illustrating the reciprocal vibration and positionof the cutting tool in the embodiment of the present invention.

FIG. 4A is a diagram illustrating the position of the cutting tool inone reciprocal vibration.

FIG. 4B is a diagram illustrating the paths traced by a cutting workposition corresponding to FIG. 4A on the basis of a workpiece.

FIG. 4C is a diagram illustrating a modified version of FIG. 4B.

FIG. 4D is a diagram illustrating a modified version of FIG. 4B.

FIG. 5A is a diagram illustrating a first modification of the repetitivemovement waveform illustrating the position of the cutting tool in onerepetitive movement.

FIG. 5B is a diagram illustrating the paths traced by a cutting workposition corresponding to FIG. 5A on the basis of a workpiece.

FIG. 5C is a diagram illustrating a second modification of therepetitive movement waveform illustrating the position of the cuttingtool in one repetitive movement.

FIG. 5D is a diagram illustrating the paths traced by a cutting workposition corresponding to FIG. 5C on the basis of a workpiece.

FIG. 6A is a diagram illustrating the position of the cutting tool inone reciprocal vibration.

FIG. 6B is a diagram illustrating the paths traced by a cutting workposition corresponding to FIG. 6A on the basis of a workpiece (areciprocal vibration waveform of the conventional art).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A machine tool and a control apparatus of the machine tool according toan aspect of the present invention may be embodied in an any manner aslong as the repetitive movement unit is configured so that the cuttingtool performs one repetitive movement with respect to multiple relativerotations between the workpiece and the cutting tool and so that arotation angle of the relative rotation during the relative movement atthe second speed is smaller than a rotation angle of the relativerotation during the relative movement at the first speed in onerepetitive movement, for making it possible to cut the workpieceefficiently while the repetitive movement due to the relative movementat the mutually different first and second speeds is performed and tolimit degradation of machining efficiency particularly when therepetitive movement constitutes a vibration.

FIG. 1 is a diagram illustrating a machine tool 100 having a controlapparatus C that is an embodiment of the present invention. The machinetool 100 includes a spindle 110 and a cutting tool rest 130A. Thespindle 110 has a chuck 120 provided at a tip thereof. A workpiece W isheld by the spindle 110 via the chuck 120, and the spindle 110 isconfigured as a workpiece holding unit to hold a workpiece. The spindle110 is supported by a spindle stock 110A so as to be rotatably driven bya spindle motor that is not shown. As the spindle motor, a conventionalbuilt-in motor or the like formed between the spindle stock 110A and thespindle 110 may be used in the spindle stock 110A.

The spindle stock 110A is mounted on a bed side of the machine tool 100so as to be movable in a Z-axis direction, which is an axis direction ofthe spindle 110, by a Z-axis direction feeding mechanism 160. Thespindle 110 moves in the Z-axis direction by the Z-axis directionfeeding mechanism 160 via the spindle stock 110A. The Z-axis directionfeeding mechanism 160 constitutes a spindle moving mechanism for movingthe spindle 110 in the X-axis direction.

The Z-axis direction feeding mechanism 160 includes a base 161, which isintegral a side on which the Z-axis direction feeding mechanism 160 isfixed, such as the bed side, and a Z-axis direction guide rail 162provided on the base 161 so as to extend in the Z-axis direction. AZ-axis direction feeding table 163 is slidably supported on the Z-axisdirection guide rail 162 via a Z-axis direction guide 164. A mover 165 aof a linear servo motor 165 is provided on the side of the Z-axisdirection feeding table 163, and a stator 165 b of the linear servomotor 165 is provided on the side of the base 161.

The spindle stock 110A is mounted on the Z-axis direction feeding table163, and the Z-axis direction feeding table 163 is driven by the linearservo motor 165 to move in the Z-axis direction. Due to the movement ofthe Z-axis direction feeding table 163, the spindle stock 110A moves inthe Z-axis direction, making the spindle 110 move in the X-axisdirection.

A cutting tool 130, such as a bite, for cutting the workpiece W isattached to the cutting tool rest 130A. The cutting tool rest 130Aconstitutes a tool rest that holds the cutting tool 130. The cuttingtool rest 130A is provided on a bed side of the machine tool 100 so asto be movable in an X-axis direction, which is perpendicular to theZ-axis direction, and in a Y-axis direction, which is perpendicular toboth the Z-axis direction and the X-axis direction, by an X-axisdirection feeding mechanism 150 and a Y-axis direction feeding mechanismthat is not illustrated. The X-axis direction feeding mechanism 150 andthe Y-axis direction feeding mechanism constitute a tool rest movingmechanism for moving the cutting tool rest 130A in the X-axis directionand the Y-axis direction with respect to the spindle 110.

The X-axis direction feeding mechanism 150 includes a base 151, which isintegral with a side on which the X-axis direction feeding mechanism 150is fixed, and an X-axis direction guide rail 152 provided on the base151 so as to extend in the X-axis direction. An X-axis direction feedingtable 153 is slidably supported on the X-axis direction guide rail 152via an X-axis direction guide 154.

A mover 155 a of a linear servo motor 155 is provided on the side of theX-axis direction feeding table 153, and a stator 155 b of the linearservo motor 155 is provided on the side of the base 151. The X-axisdirection feeding table 153 is driven by the linear servo motor 155 tomove in the X-axis direction. The Y-axis direction feeding mechanism isstructurally similar to the X-axis direction feeding mechanism 150,except being arranged in the Y-axis direction. Thus, the detaileddescription and illustration of the Y-axis direction feeding mechanismare omitted.

In FIG. 1, the X-axis direction feeding mechanism 150 is mounted on thebed side via the Y-axis direction feeding mechanism that is not shown,and the cutting tool rest 130A is mounted on the X-axis directionfeeding table 153. The cutting tool rest 130A moves in the X-axisdirection by being driven by the X-axis direction feeding table 153, andalso moves in the Y-axis direction by being driven by the Y-axisdirection feeding mechanism, which operates similarly to the X-axisdirection feeding mechanism 150.

Alternatively, the Y-axis direction feeding mechanism that is not shownmay be mounted on the bed side via the X-axis direction feedingmechanism 150, and the cutting tool rest 130A may be mounted on the sideof the Y-axis direction feeding mechanism. The structure for moving thecutting tool rest 130A in the X-axis direction and the Y-axis directionby the X-axis direction feeding mechanism and the Y-axis directionfeeding mechanism 150 is conventionally known and thus the detaileddescription and illustration of the structure are omitted.

The tool rest moving mechanism (the X-axis direction feeding mechanism150 and the Y-axis direction feeding mechanism) and the spindle movingmechanism (the Z-axis direction feeding mechanism 160) operatecooperatively, and the cutting tool 130 attached to the cutting toolrest 130A is fed in any feeding direction with respect to the workpieceW via the movement of the cutting tool rest 130A in the X-axis directionand the Y-axis direction by the X-axis direction feeding mechanism 150and the Y-axis direction feeding mechanism as well as via the movementof the spindle stock 110A (the spindle 110) in the Z-axis direction bythe Z-axis direction feeding mechanism 160.

As illustrated in FIG. 2, the workpiece W is cut with the cutting tool130 into any shape by feeding the cutting tool 130 in any feedingdirection with respect to the workpiece W by a feeding unit consistingof the spindle moving mechanism (the Z-axis direction feeding mechanism160) and the tool rest moving mechanism (the X-axis direction feedingmechanism 150 and the Y-axis direction feeding mechanism).

In this embodiment, both the spindle stock 110A and the cutting toolrest 130A are movable. Alternatively, the spindle stock 110A may befixed on the bed side of the machine tool 100 and the tool rest movingmechanism may be configured to move the cutting tool rest 130A in theX-axis direction, the Y-axis direction, and the Z-axis direction. In thelatter case, the feeding unit may be consist of the tool rest movingmechanism that moves the cutting tool rest 130A in the X-axis direction,the Y-axis direction, and the Z-axis direction, and the cutting tool 130may be fed toward the workpiece W by moving the cutting tool rest 130Awith respect to the spindle 110 that is fixedly positioned and rotatablydriven.

Also, the cutting tool rest 130A may be fixed on the bed side of themachine tool 100 so as to be not movable and the spindle movingmechanism may be configured so as to move the spindle stock 110A in theX-axis direction, the Y-axis direction, and the Z-axis direction. Inthis case, the feeding unit may be consist of the spindle movingmechanism that moves the spindle stock 110A in the X-axis direction, theY-axis direction, and the Z-axis direction, and the cutting tool 130 maybe fed toward the workpiece W by moving the spindle stock 110A withrespect to the cutting tool rest 130A that is fixedly positioned.

Although the X-axis direction feeding mechanism 150, the Y-axisdirection feeding mechanism, and the Z-axis direction feeding mechanism160 are configured to be driven by a linear servo motor in thisembodiment, they may be driven by a conventional mechanism consisting ofa ball screw and a servo motor, for example.

In this embodiment, a rotating unit to relatively rotate the workpiece Wand the cutting tool 130 consists of the spindle motor such as thebuilt-in motor, and the relative rotation between the workpiece W andthe cutting tool 130 is achieved by rotatably driving the spindle 110.Although the present embodiment is configured so that the workpiece W isrotated with respect to the cutting tool 130, the cutting tool 130 maybe rotated with respect to the workpiece W. In the latter case, arotating tool such as a drill may be used as the cutting tool 130. Therotation of the spindle 110, the Z-axis direction feeding mechanism 160,the X-axis direction feeding mechanism 150, and the Y-axis directionfeeding mechanism are driven and controlled by a control part C1 of thecontrol apparatus C. The control part C1 is preconfigured to control sothat the spindle stock 110A or the cutting tool rest 130A moves in anyone of the X-axis direction, the Y-axis direction, and the Z-axisdirection while the spindle 110 and the cutting tool 130 reciprocallyvibrate in a relative manner as one example of the repetitive movementby repeating a relative movement between the spindle 110 and the cuttingtool 130 at a first speed and a relative movement between the spindle110 and the cutting tool 130 at a second speed, which is different fromand slower than the first speed, in the any one of the X-axis direction,the Y-axis direction, and the Z-axis direction by utilizing one of thefeeding mechanisms as a repetitive movement unit.

As illustrated in FIG. 3, due to the control of the control part C1,each of the feeding mechanisms forwardly moves the spindle 110 or thecutting tool rest 130A (forward movement) for a predetermined forwardmovement amount as the relative movement at the first speed and thenbackwardly moves the spindle 110 or the cutting tool rest 130A (backwardmovement) for a predetermined backward movement amount as the relativemovement at the second speed in each reciprocal vibration, so that thespindle 110 or the cutting tool rest 130A moves in a respectivedirection for an advancing amount that is equal to the differencebetween the forward movement amount and the backward movement amount. Bydoing so, the feeding mechanisms cooperatively feed the cutting tool 130toward the workpiece W in any feeding direction.

As illustrated in FIG. 4A, due to the Z-axis direction feeding mechanism160, the X-axis direction feeding mechanism 150, and the Y-axisdirection feeding mechanism, the machine tool 100 machines the workpieceW by feeding the cutting tool 130 in any feeding direction on the basisof a feed amount, which is a part of the advancing amount achieved inone rotation of the spindle, or while the phase of the spindle changesfrom zero to 360 degrees. The control part C1 of the present embodimentcontrols the feeding unit so that the rotation angle of the spindle 110during the backward movement is smaller than the rotation angle of thespindle 110 during the forward movement in one reciprocal vibration.

If the spindle stock 110A (spindle 110) or the cutting tool rest 130A(cutting tool 130) moves while reciprocally vibrates in accordance withthe reciprocal vibration waveform illustrated in FIG. 4A while theworkpiece W rotates for machining the workpiece W with the cutting tool130 into a predetermined shape, the workpiece W is cut along a path a orpath b in each rotation of the workpiece W as illustrated in FIG. 4B.

One example will now be described in which the number of vibrations N ofthe spindle stock 110A (spindle 110) or the cutting tool rest 130A withrespect to one rotation of the workpiece W is 0.5 (the number ofvibrations N=0.5), as illustrated in FIG. 4A.

As illustrated in FIG. 4A, the control part C1 controls the feeding unitso that the workpiece W is machined while the cutting tool 130 performsone reciprocal vibration for each two rotations of the spindle 110(workpiece W).

In the present embodiment, the control part C1 controls so that therotation angle of the spindle 110 during the backward movement of thecutting tool 130 is smaller than the rotation angle of the spindle 110during the forward movement of the cutting tool 130 in one reciprocalvibration. In other words, the cutting tool 130 moves forward while thespindle 110 rotates 540 degrees and then moves backward while thespindle 110 rotates another 180 degrees so that, at the end of thesecond rotation of the spindle 110, the cutting work position of theforward movement of the cutting tool 130 comes into contact and overlapswith the cutting work position of the backward movement of the cuttingtool 130.

Because of this, the cutting work position of the backward movement ofthe cutting tool 130 is theoretically included in the cutting workposition of the forward movement of the cutting tool 130 as a “point” ineach reciprocal vibration of the cutting tool 130. This makes an “aircut”, in which the cutting tool 130 leaves the workpiece W, occur as a“point” in the backward movement of the cutting tool 130. Thus, a chipgenerated from the workpiece W during a machining operation issequentially separated due to the air cut (that occurs at the pointwhere the cutting work position of the forward movement of the cuttingtool 130 comes into contact with the cutting work position of thebackward movement of the cutting tool 130). Thus, the machine tool 100can smoothly machine an outer surface of the workpiece W whileseparating a chip by the reciprocal vibration of the cutting tool 130 ina feeding direction.

Like the reciprocal vibration waveform illustrated in FIGS. 4A and 4B,the cutting work position of the forward movement comes into contactwith the cutting work position of the backward movement at anintersection point CT between the path a traced in the first rotation ofthe spindle and the path b traced in the second rotation of the spindle.Thus, the air cut occurs as a “point”, making a chip being separatedinto chip powder.

The amplitude of the reciprocal vibration illustrated in FIGS. 4A and 4Bwill now be described. Firstly, due to the preset feed amount of thecutting tool 130 toward the workpiece W, the start point of the path atraced by the cutting tool 130 in the first rotation of the spindle andthe end point of the path b traced by the cutting tool 130 in the lastof the multiple rotations (in this case, the second rotation) of thespindle are determined.

Next, an amplitude amount, which is the amount of backward movement ofeach reciprocal vibration in a feeding direction, is calculated and setso that the absolute value of a forward movement speed of the cuttingtool 130 and the absolute value of a backward movement speed of thecutting tool 130 become equal to each other when the movement of thecutting tool 130 switches from forward to backward at 180 degrees in thelast of the multiple rotations (in this example, the second rotation) ofthe spindle 110 that are performed while the cutting tool 130 performsone reciprocal vibration. In other words, it is calculated and set sothat a part of the path b traced in the forward movement and a part ofthe path b traced in the backward movement constitute two sides of equallength of an isosceles triangle. When the amplitude amount iscalculated, the path a traced by the cutting tool 130 and the path bbraced by the cutting tool 130 are determined.

FIGS. 4A and 4B illustrate an example in which the origin of the spindle(the position of the spindle 110 at zero degree) predetermined as areference for controlling the rotation of the spindle 110, for example,is used as a reference angle position and in which the cutting tool 130starts a cutting work from the origin of the spindle.

The cutting tool 130 starts the reciprocal vibration from the origin ofthe spindle (zero degree). After moving forward while the spindle 110performs a first rotation from the origin of the spindle, the cuttingtool 130 switches its movement from forward to backward during a secondrotation of the spindle 110 from another origin of the spindle. At theend of this second rotation, or when the spindle 110 reaches yet anotherorigin of the spindle, the cutting work position of the backwardmovement reaches and comes into contact with the cutting work positionof the forward movement, completing one reciprocal vibration. Thecutting tool 130 then starts next reciprocal vibration from the yetanother origin of the spindle. As a result, the cutting tool 130 doesnot continue the backward movement more than necessary from the point atwhich an air cut occurs, making it possible to increase machiningefficiency.

The reference angle position is not necessarily the origin of thespindle and may be a predetermined rotation angle position of thespindle 110. The cutting work position of the backward movement reachesand comes into contact with the cutting work position of the forwardmovement at the reference angle position. In other words, the rotationangle position of the spindle 110 corresponding to the intersectionpoint CT is set as the reference angle position. Even if the cuttingwork position of the backward movement comes into contact and overlapswith the cutting work position of the forward movement at the referenceangle position as illustrated in FIGS. 4A and 4B, the cutting workposition of the backward movement may not sufficiently come into contactwith the cutting work position of the forward movement in an actualcutting work. Thus, as illustrated in FIGS. 4C and 4D, it is alsopossible to make the cutting tool 130 perform the reciprocal vibrationwhile a predetermined overlap timing adjustment angle is set so that theintersection point CT is located at a predetermined rotation angleposition of the spindle 110 before reaching the reference angleposition.

For example, the predetermined overlap timing adjustment angle can beset by changing the amplitude amount as illustrated in FIG. 4C. Also,the predetermined overlap timing adjustment angle can be set by reducingthe phase of the spindle at which the cutting tool 130 switches itsmovement from forward to backward within the second rotation of thespindle without changing the amplitude amount of one reciprocalvibration as illustrated in FIG. 4. This makes the cutting work positionof the forward movement overlaps with the cutting work position of thebackward movement for the length of the overlap timing adjustment angle.The cutting tool 130 then performs an air cut in this overlappingperiod. Therefore, a chip can be reliably separated.

In this case, the reference angle position is determined as a rotationangle position of the spindle 110 calculated by adding the overlaptiming adjustment angle to the rotation angle position corresponding tothe intersection point CT.

Change of the phase of the spindle at which the cutting tool 130switches its movement from forward to backward without changing theamplitude amount of one reciprocal vibration can be executed on thebasis of a ratio between the rotation amount of the spindle with respectto one reciprocal vibration of the cutting tool 130 and the rotationamount of the spindle during the backward movement of the cutting tool130. When the amplitude amount is set as illustrated in FIG. 4A and thecutting tool 130 switches its movement from forward to backward at aposition between zero to 180 degrees in the last of the multiplerotations (in this example, the second rotation) of the spindle 110performed while the cutting tool 130 performs one reciprocal vibration,the rotation amount of the spindle during the backward movement can bechanged between 0.5 to 1.0 (180 to 360 degrees) without changing theamplitude amount. That is, as the rotation amount of the spindle duringthe backward movement increases, the apex of the path b of the cuttingtool 130 illustrated in FIG. 4D moves left, making it possible tooverlap the cutting work position of the forward movement with thecutting work position of the backward movement.

In this case, the slope of the path a of the cutting tool 130 becomessteeper and the right slope of the path b with respect to the apex(backward movement path) becomes gentler without changing the locationof the end point of the path b of the cutting tool 130. Thus, theintersection point CT will be located between the apex of the path b andthe end point of the path b.

If the control part C1 is configured so that the start of a vibrationalcutting work, in which the cutting tool 130 is fed in a feedingdirection while reciprocally vibrating in the feeding direction in arelative manner with respect to the workpiece W, is instructed in amachining program with a command G***P2, a value of the number ofvibrations set to the control part C1 can be specified as the number ofrotations of the spindle 110 with respect to one reciprocal vibration byusing a value of E (argument E) succeeding the command G***P2, and therotation amount of the spindle during the backward movement set to thecontrol part C1 can be specified by using a value of R (argument R).

Because the cutting tool 130 switches to the backward movement after theforward movement is performed while the workpiece W rotates multipletimes, cutting efficiency can be increased by increasing the feed amountof the cutting tool 130. Also, by increasing the rotation amount of theworkpiece W during the forward movement and decreasing the rotationamount of the workpiece W during the backward movement, a cutting loadacting on the cutting tool 130 can be reduced.

In the present embodiment, the number of vibrations N is set so thatless than one reciprocal vibration is performed with respect to onerotation of the spindle 110 (workpiece W) (0<the number of vibrationsN<1.0).

In the machine tool 100, an operation instruction by the control part C1is executed at predetermined instruction cycles. The reciprocalvibration of the spindle stock 110A (spindle 110) or the cutting toolrest 130A (cutting tool 130) can be executed at predeterminedfrequencies that are based on the instruction cycles. The instructioncycles are determined on the basis of a reference cycle and aregenerally the integral multiples of the reference cycle.

The reciprocal vibration can be executed at frequencies that are inaccordance with values of the instruction cycles. A frequency (vibrationfrequency) f (Hz) of the reciprocal vibration of the spindle stock 110A(spindle 110) or the cutting tool rest 130A (cutting tool 130) is set toa value selected from the frequencies.

When the spindle stock 110A (spindle 110) or the cutting tool rest 130A(cutting tool 130) reciprocally vibrates and the number of rotations ofthe spindle 110 is S (r/min), then the number of vibrations N can becalculated as N=f*60/5. The number of rotations S is inverselyproportional to the number of vibrations N with the vibration frequencyf as a constant.

The spindle 110 can be rotated at a faster speed by increasing thevibration frequency f or decreasing the number of vibrations N. If thenumber of vibrations N is less than 1.0, then the spindle 110 can berotated at a high speed without decreasing cutting efficiency bycontrolling the reciprocal vibration with the control apparatus C.

As described above, the repetitive movement unit is configured so thatthe cutting tool 130 performs one repetitive movement with respect tomultiple rotations of the spindle 110 and so that the rotation angle ofthe spindle 110 during the backward movement is smaller than therotation angle of the spindle 110 during the forward movement in onerepetitive movement. Thus, the machine tool 100 and the controlapparatus C of the machine tool 100 as an embodiment of the presentinvention are capable of limiting degradation of machining efficiencywhen the cutting tool machines the workpiece W by performing onereciprocal vibration with respect to multiple rotations of the spindle110.

Also, the reference angle position is set on the basis of a rotationangle position of the spindle 110 at which the cutting work position ofthe backward movement and the cutting work position of the forwardmovement intersect with each other, and the repetitive movement unit isconfigured so that, during one rotation from the reference angleposition after the forward movement is performed for the length of apredetermined number of rotations of the spindle 110 from the referenceangle position, the forward movement switches to the backward and thecutting work position of the backward movement reaches the cutting workposition of the forward movement to complete one vibration. Thus, it ispossible to minimize a non-productive operation in a cutting work toincrease machining efficiency

Also, rather than repeating the forward movement as the relativemovement at the first speed and the backward movement as the relativemovement at the second speed as illustrated in FIG. 4A, the second speedmay be set to zero to stop the relative movement in a feeding directionas illustrated in FIG. 5A. In this case, in comparison with thereciprocal vibration illustrated in FIG. 4A, it is possible to increasecutting efficiency by increasing the feed amount.

As illustrated in FIG. 5B, although the path a traced in the firstrotation of the spindle does not intersect with the path b traced in thesecond rotation of the spindle when the second rotation completes, thedistance between the path a and the path b becomes smaller. Thus, a chipgenerated from the workpiece W becomes narrower at this point, so thatthe chip can be easily folded and separated at this point into chippowder.

Also, rather than repeating the forward movement as the relativemovement at the first speed and the backward movement as the relativemovement at the second speed as illustrated in FIG. 4A, the relativemovement at the second speed may be performed in the same direction asthe relative movement at the first speed while the second speed is setslower than the first speed, as illustrated in FIG. 5C. In this case, incomparison with the repetitive movement illustrated in FIG. 5A, it ispossible to increase cutting efficiency by increasing the feed amount.

As illustrated in FIG. 5D, although the path a traced in the firstrotation of the spindle does not intersect with the path b traced in thesecond rotation of the spindle when the second rotation completes, thedistance between the path a and the path b becomes smaller, as with thecase illustrated in FIG. 5B. Thus, a chip generated from the workpiece Wbecomes narrower at this point, so that the chip can be easily foldedand separated at this point into chip powder.

What is claimed is:
 1. A machine tool comprising: a workpiece holdingunit to hold a workpiece; a tool rest to hold a cutting tool for cuttingthe workpiece; a feeding unit to feed the cutting tool toward theworkpiece in a predetermined feeding direction via relative movementbetween the workpiece holding unit and the tool rest; a repetitivemovement unit to move the workpiece holding unit and the tool rest in amutually relative manner in repeated cycles of relative movement of saidworkpiece holding unit and said tool rest along said predeterminedfeeding direction, each of said repeated cycles consisting of a relativemovement of said workpiece holding unit and said tool rest at a firstspeed and a relative movement of said workpiece holding unit and saidtool rest at a second speed different from said first speed, said firstand second speeds of relative movement being in a range includingrelative movement at a speed of zero; and a rotating unit to relativelyrotate the workpiece and the cutting tool; wherein the machine tool iscapable of machining the workpiece via the relative rotation between theworkpiece and the cutting tool and via the feeding of the cutting tooltoward the workpiece with said repeated cycles of relative movement; andwherein the repetitive movement unit is configured so that the cuttingtool performs one repeated cycle with respect to multiple relativerotations between the workpiece and the cutting tool and so that arotation angle of the relative rotation during the relative movement atsaid second speed is smaller than a rotation angle of the relativerotation during the relative movement at the first speed in each of saidrepeated cycles.
 2. The machine tool according to claim 1, wherein saidfirst speed is greater than said second speed.
 3. The machine toolaccording to claim 1, wherein said second speed is zero.
 4. The machinetool according to claim 1, wherein said first speed is greater than saidsecond speed, and wherein said relative movements at said first speedand at said second speed are in the same direction.